Technical Practices and Reflections on the Transformation of the Metallurgical Industry’s Rolling Mill Control System Using Siemens S7-400

A stable automation control system is like a silent guardian of the factory; it does not need a glamorous appearance but must have a strong and reliable core.

Origin: An Imminent Transformation

Can you imagine the scene when a hot-rolled production line with a daily output of 8,000 tons suddenly stops? 🔥

That was in the summer of 2019. I had just returned to the company after finishing another project when I received an urgent call. A steel plant in Jiangsu had a serious failure in its 1580 hot-rolled production line. The control system, which had been in use for 15 years, frequently crashed, causing the rolling mill to operate abnormally. The loss from downtime reached up to 300,000 yuan per hour, and the plant leaders were as anxious as ants on a hot pan.

“Old Zhang, we really need you this time!” The project manager patted my shoulder, “The client requires the system upgrade to be completed within a month without affecting normal production.”

A month? Normally, such a large-scale transformation would take at least three months! But seeing the anxious look in the project manager’s eyes, I nodded. This is the daily life of an industrial automation engineer—finding possibilities in the impossible.

System Hardware Configuration: Stability is the Top Priority

Considering the high reliability requirements of the rolling mill control system, we chose the best of the Siemens S7-400 series—CPU 416-3 PN/DP as the core control unit. This CPU has 16MB of working memory, sufficient to handle complex rolling control algorithms.

The hardware configuration list is as follows:

• Central Processing Unit: CPU 416-3 PN/DP × 2 (Redundant configuration)
• Power Supply Module: PS 407 20A × 4
• Communication Processor: CP 443-1 Advanced × 4 (Profinet communication)
• Remote I/O Station: ET 200M × 12 (Distributed across various racks)
• Analog Input Module: SM 331 × 24 (For measuring temperature, pressure, position, etc.)
• Analog Output Module: SM 332 × 16 (For controlling servo valves, inverters, etc.)
• Digital I/O Module: SM 321/322 × 48 (For processing switch signals)

Do you know why we chose a redundant configuration? Because in a rolling mill, system downtime means huge economic losses and could even cause equipment damage. The redundant configuration ensures that even if one CPU fails, the other can seamlessly take over, ensuring production continuity.

System Design: The Art of Control Architecture

What is the biggest challenge in designing a rolling mill control system? That’s right, it’s the balance between real-time performance and reliability.

We adopted a three-layer control architecture:

1. Basic Automation Layer: S7-400 PLC is responsible for all I/O signal processing and basic control logic
2. Process Control Layer: Implements core algorithms for rolling force control, tension control, speed control, etc.
3. Coordinated Control Layer: Achieves coordinated control between various rolling mills to ensure product quality

The most critical part is the rolling force control system. Traditional PID control struggles with rapidly changing rolling conditions, so we added a feedforward compensation step. By measuring the thickness of the incoming steel plate, we can calculate the required rolling force in advance, which is then precisely adjusted by the PID controller. This control strategy improved thickness accuracy by 35%, achieving a level of ±0.05mm.

Have you ever encountered a situation where the theoretically sound control strategy does not perform well in practice? 🤔 I have too. Therefore, we paid special attention to the fault handling mechanism during the design, setting up multiple levels of protection for each critical control loop.

Program Implementation: Craftsmanship in Code

Speaking of program implementation, I want to share a little story. At the beginning of the project, a young engineer on the team suggested writing all programs in SCL language, reasoning that it was “more efficient and modern.” I asked him, “If a problem arises at 3 AM on-site and the operator only understands ladder diagrams, do you think he can fix a program written in SCL?”

We adopted the following program structure:

• OB1: Main loop, calls various function blocks
• OB35: 100ms cycle interrupt for PID control algorithm
• OB82-OB86: Diagnostics and fault handling
• FB100-FB199: Basic functions of the rolling mill (motor control, valve control, etc.)
• FB200-FB299: Rolling process control (thickness control, flatness control, etc.)
• FC300-FC399: Auxiliary functions (calculations, conversions, communications, etc.)
• DB500-DB999: Data blocks, storing parameters and intermediate variables

The key control algorithms were written in SCL, while the device control logic was implemented using ladder diagrams. This ensured both efficient implementation of algorithms and convenience for on-site maintenance.

A little tip: We marked the “last modified by” and “last modification reason” in the comments of each function block. This may seem simple, but it saves a lot of time during later maintenance.

Debugging Experience: From “Running” to “Running Stably”

After the system development was completed, the most challenging phase for engineers arrived—debugging. We developed a detailed commissioning plan:

1. Simulation Testing: Comprehensive simulation testing using PLCSIM
2. On-site I/O Check: Point-by-point check of all input and output signals
3. No-load Debugging: Equipment linkage testing without steel plates
4. Load Testing: Gradually increasing output and adjusting control parameters
5. Performance Optimization: Analyzing operational data and optimizing control parameters

The biggest challenge during this period was solving communication fluctuation issues. In the high electromagnetic interference environment of the rolling mill, the Profinet network occasionally experienced communication interruptions. We tried various solutions and ultimately resolved the issue through the following measures:

• Using fiber optics instead of copper cables to connect key nodes
• Adding network isolation transformers
• Optimizing network topology to reduce branching levels
• Adjusting the retransmission parameters of CP 443-1

Note: The communication cycle settings in the Profinet network are crucial and must be configured reasonably according to the response time requirements of the control loop to avoid control performance degradation due to communication delays.

Project Benefits: Data Speaks

After the transformation, the system stability significantly improved, with no unplanned downtime due to control system failures within a year. The specific benefits are reflected in:

• Improved Product Quality: Thickness accuracy increased by 35%, flatness qualification rate improved by 18%
• Increased Production Efficiency: Rolling speed increased by 15%, reaching 10m/s
• Reduced Energy Consumption: Power consumption per ton of steel reduced by 8.5%
• Lower Maintenance Costs: Annual maintenance costs reduced by approximately 400,000 yuan

What makes me most proud is that after this transformation, the client finally freed themselves from dependence on foreign engineers and fully mastered the system maintenance capabilities.

Insights: The Warmth of Technology

Looking back on this project, my biggest realization is that the success of an automation system depends not only on technology selection but also on the deep integration with processes.

We engineers often focus on the precision and response speed of control algorithms while neglecting the user experience of operators. An excellent system should be like air—present but unnoticed; only when problems arise do people realize its importance.

If you are undertaking a similar transformation project, my advice is to spend more time communicating with on-site operators to understand their pain points and needs. No matter how advanced the technology is, if it cannot solve practical problems, it is just empty talk.

Do you have similar project experiences? Feel free to share your stories and insights in the comments. On the road of industrial automation, we are all travelers and companions.

Remember, every system upgrade is not just an update of equipment but an improvement of process levels and a redefinition of the relationship between people and machines. By putting heart into every project, technology can truly warm people’s hearts.

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